ELSEVIER SSDI 0031-9384(95)02272-4 Physiology & Behavior, Vol. 6C. No. I, 139-143, 1996 Copyright © 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 00310384/96 $15,00 + .00 Sexual Behavior in Male Rats After Nitric Oxide Synthesis Inhibition MICHAL BIALY, I JOZEF BECK, PIOTR ABRAMCZYK, ANDRZEJ TRZEBSKI AND JACEK PRZYBYLSKI Department of Human Physiology, Medical School, Krakowskie Przedmie~cie 26/28, 00-325 Warsaw, Poland Received 15 May 1995 BIALY, M., J. BECK, P. ABRAMCZYK, A. TRZEBSKI AND J. PRZYBYLSKI. Sexual behavior in male rats after nitric oxide synthesis inhibition. PHYSIOL BEHAV 60(1) 139-143, 1996.--The influence of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) on the copulatory behavior of sexually experienced male Wistar rats was investigated. L-NAME was injected IP 10 min before the onset of a session using a dose of 30 mg/kg (L-NAME 30 group), or 60 mg/kg (L-NAME 60 group). The copulatory sessions were terminated after the third ejaculation in the control group or after 1500 s in the L-NAME 30 and L-NAME 60 groups. L-NAME administration reduced the number of rats that achieved ejaculation by 43% and 86% in the L-NAME 30 and 60 groups, respectively. In both experimental groups only a few intromissions and an increased number of mountings were observed. An increase in the number of ultrasonic vocalizations in the 50 kHz band, a dose-dependent effect, was observed. The level of sexual motivation evaluated by mount latency was not influenced by inhibition of NO synthesis. Sexual behavior Nitric oxide Rat IN 1863 Eckhard demonstrated that the erectile process is trig- gered by the autonomic nervous system (8). Since then, extensive research has been devoted to identifying the specific mechanisms responsible for this phenomenon. During the last decade transmit- ters such as acetylcholine and VIP were implicated in the erectile process (1). Recently, the role of nitric oxide (NO) in the erection process has attracted considerable attention (15,17,27,28,32). Ig- narro et al, (13) have shown that the relaxation of isolated strips of rabbit corpus cavernosum, due to field stimulation, was abol- ished by adding inhibitors of NO synthesis. On the other hand, exogenous donors of NO caused a marked relaxation of the isolated rabbit corpora cavernosa (11). Pretreatment with L-NNA abolished the increase in blood pressure within the rat corpora cavernosa produced by electrical stimulation of the lumbo-sacral segment of the spinal cord (9,25,26). These observations were confirmed on human corpora cavernosa obtained from the ampu- tated penis of men without a previous history of impotency (6). NO synthase, the enzyme responsible for NO generation, has been localized in neurons in the major pelvic ganglion and varicose nerve terminals associated with the corpora cavernosa both in rats and humans (1,5,14). Recently, Hull et al. (12) observed that NAME decreased the number of erections during copulatory behavior as well as during the "ex copula" test. Moreover, on the basis of X-maze tests, they found that NAME did not influence sexual motivation. The aim of the present study was to ascertain if, and how, the NO synthase inhibitor affects the sexual behavior of male rats, by using as an indicator not only the analysis of the copulatory behavior but also the number of ultrasonic vocalizations in the 50 kHz band. METHOD Twenty-one male and 10 female albino rats of Wistar strain were used. The animals were 16 weeks old at the beginning of the study. They were maintained on reversed 12 L:I2 D cycle, with light off from 0900 to 2100 h. They were housed three per standard laboratory cage. Before the onset of the experiment each male achieved six ejaculations in six preexperimental (training) sessions, which was shown previously to be enough to reach sexual experience (4,18). The behavioral estrus was induced in previously spayed females by SC injections of 25 /~g estradiol benzoate and 500 /xg progesterone, 48 and 3-6 h before the test, respectively. N-Nitro-L-arginine methyl ester (L-NAME; Sigma) was administered 1P 10 min before the session at a dose of 30 mg/kg (n= 7) or 60 mg/kg (n= 7). In the control group (n = 7) only 0.9 NaC1 was injected. Mating tests were conducted in a glass arena measuring 55 x 65 × 20 cm. The floor of the arena was covered with absorbent paper. This paper was changed after each session. t To whom requests for reprints should be addressed. 139